console.log(null == undefined)     // true
console.log(null === undefined)    // false


console.log(typeof (1 + null))  // number
console.log(1 + null)          // 1
console.log(typeof (1 + undefined))  // number
console.log(1 + undefined)           //

console.log(isNaN(1 + null))  // false
console.log(isNaN(1 + undefined)) // true


console.log(null == null)
console.log(null === undefined)
console.log(undefined == undefined)
console.log(undefined === undefined)


console.log(undefined === undefined)
console.log(undefined === null)


console.log(Symbol('fn') == Symbol('fn'))

console.log(1 + undefined)
console.log(isNaN(1 + undefined)) // true
console.log(1 + null)
console.log(isNaN(1 + null)) // false
